In such types of shears, the hand levers are joined to one another directly, independently of the cutting or shearing elements, by means of a second joint which is offset parallel to the cutting element joint and in which one of the cutting elements is connected rigidly with one of the two hand levers, while the other shearing blade has a driving arm which protrudes beyond the shearing blade joint and to which one end of a coupler is hinged. The other end of the coupler, also freely movable, is fastened to the second hand lever, the two hand levers and the two cutting elements forming a four-joint system with one another.
"Cutting elements" are understood to be, in a first type of shears, two shearing blades which work against one another and, in a second type of shears, an anvil and a cutting edge working against this anvil.
Hand shears of the first named type are already known and, compared to shears constructed according to a one joint system, have the considerable advantage that they make power transmission possible, which depends not only on the ratio of the length of the arms of the cutting elements on the one hand and of the hand levers carrying these cutting elements on the other, but is also determined significantly by the length ratio of the individual elements of the four-joint system.
Appreciable cutting forces can readily be achieved by applying relatively small forces by hand to the operating handles of the two hand levers.
However, German Offenlegungsschrift 25 08 973 also discloses hand shears of the same type of construction, in which, with little additional technical effort for different applications or uses, optimization in each case of the power transmission between the hand levers and the cutting elements can be achieved.
Such a change in the power transmission is desirable, for example, when the hand shears are used to cut materials, the cross-sectional dimensions of which differ significantly from case to case.
This advantage is achieved in the known device owing to the fact that at least the distance between the linking axis of the coupler on the second hand lever and the joint directly linking the two hand levers is variable.
By carrying out adjusting measures at a single place of the four-joint system, the available adjustment path for the two hand levers can be varied without there being any change in the available regulating distance between the two shearing blades.
The changing of the power transmission in the known hand shears is however awkward and time consuming because, for this purpose, it is always necessary to loosen the hinge pin for the coupler from one borehole and subsequently to transfer it into the other borehole. At the same time, it is essential to secure the pin in its working position against inadvertent loosening, otherwise it can be lost and/or the hand shears are unable to function.